Is the system working at this time? If all it needs is the refrigerant charge topped off. Don't convert. If you have a mafor problem with the system then it is probably worth converting. The old oil has to be drained out of the system. New oil that is R134 compatible added, change the Receiver/Dryer. and charge with r134. Some will tell you to change all the seales in the system. I have converted several of my systems. and only changed seals where I opened the system for repairs. On of thse systems is on it's 4th year with out a problem.